Exploration, production, and monitoring of hydrocarbon and water deposits entails the measurement of subsurface characteristics and the evaluation of the obtained data to determine petrophysical properties of interest for the relevant formation or reservoir. A variety of techniques have been developed to measure subsurface characteristics. These techniques involve the subsurface deployment (usually through a borehole traversing the formations) of several different measurement and telemetry systems to provide data regarding the subsurface characteristics of interest, and data regarding the state of tools or instruments disposed downhole.
Among the various data collection and logging techniques routinely employed, systems for obtaining measurement data while drilling have proven to be cost effective. Logging While Drilling (LWD) and Measurement While Drilling (MWD) techniques are well known in the art. Logging While Tripping (LWT) systems have also been developed as an alternative to LWD and MWD techniques. In LWT, a small diameter “run-in” tool is sent downhole through the drill pipe, at the end of a bit run, just before the drill pipe is pulled. The run-in tool is used to measure downhole characteristics as the drill string is extracted or tripped out of the hole. In these types of systems, obtained data is either stored in downhole memory for later processing or transmitted to the surface using telemetry means, such as mud flow telemetry devices in the case of LWD/MWD systems.
Regardless of the conveyance means used for downhole tools, a shared requirement of the various measurement and telemetry systems is the need for electrical power. With the exception of wireline systems, it is difficult to convey electric power from the surface through the conveyance means to the components of the logging tools or the telemetry means. In these cases, electrical power can however be provided by downhole sources. Conventional systems obtain such power downhole either from a battery pack or a turbine-based alternator. Battery packs provide an energy storage medium. When using batteries, electrical power is made available until the battery is depleted. Turbine-based alternators provide an energy conversion device. In this case, electrical power is made available only when the energy source to be converted into electrical power is present.
Examples of alternators used in downhole logging tools are described in U.S. Pat. Nos. 5,517,464 and 5,793,625. An example of an alternator-like electrical torque-generator is described in U.S. Pat. No. 5,265,682. Turbine-based alternators employ rotors having impellers that are placed in the high-pressure flow of drilling fluid (“mud flow”) inside the drill string so that the impeller blades convert the hydraulic energy of the drilling fluid into rotation of the rotor. The rotor rotates at an angular velocity that provides enough energy to power the telemetry means and/or other components (e.g., sources/sensors) in the telemetry tool, and in some cases other tools in the downhole assembly.
Examples of battery packs used in downhole logging tools are described in U.S. Pat. Nos. 6,187,469 and 6,575,248. An example of a testing while drilling tool powered at least in part by a battery module disposed in the tool collar is described in U.S. Pat. No. 7,124,819 (assigned to the present assignee and entirely incorporated herein by reference). Battery packs are charged at the surface and provide electrical power to a single tool. Some batteries packs may be recharged downhole.
FIG. 1 shows a conventional land-based drilling rig 10 with connected drill pipe leading into a borehole 12 drilling through a subterranean formation F. At the tip of the drill string 14 is a drill bit 16 followed by a bottom hole assembly (BHA) 18 comprised of drilling, telemetry, and MWD/LWD tools. The borehole 12 is formed by rotary drilling in a manner that is well known. Drilling fluid or mud is pumped to the interior of drill string 14 to flow downwardly through the string. The drilling fluid exits drill string 14 via ports in the drill bit 16, and then circulates upwardly through the annular space between the outside of the drill string and the wall of the borehole as indicated by the arrows. These conventional systems are powered both in a distributed fashion, wherein an individual tool in the string contains its own battery, and in a centralized fashion, wherein an individual tool draws power from a downhole turbine.
FIG. 2 shows a conventional design for supplying power in a downhole system such as the drilling assembly of FIG. 1. Each tool in the BHA has its own dedicated battery 20. A single conductor, combined low electrical power and low speed communications bus 22 is used through all tools in the string. A low electrical power source is provided by a turbine, usually disposed in the telemetry tool, and energized by the mud flow. It should be appreciated that the bus 22 does not provide electrical coupling between the battery disposed in one tool and electrical components disposed in another tool. This configuration offers restricted spacing (a large spacing) between sources/sensors 24 in the string and presents handling and reliability issues. Thus a need remains for improved power distribution techniques for subsurface systems.